Colorants used in inkjet recording inks are roughly divided into dyes and pigments. Compared with inkjet recording inks making use of dyes as colorants, inkjet recording inks making use of pigments as colorants are characterized in that they can provide prints with excellent fastness such as waterproofness and light fastness. Dye inks are used with dyes dissolved as colorants in ink media, while pigment inks are used in the form of pigment dispersions with pigments finely dispersed as colorants in ink media. In a production process of a pigment ink, a dispersion step is, therefore, essential to finely disperse a pigment.
Especially, inkjet recording is a printing process that ink droplets are ejected through at least one very small nozzle of several tens micrometers to record an image. It is, therefore, necessary that the ink does not clog the nozzle, the ejection stability of the ink is excellent, and the ink shows high color developing ability and gloss on a recording medium. Moreover, it is also needed that the ink retains these properties over a long period, in other words, the ink has high storage stability. The division of a pigment into fine particles is essential for providing a printer with satisfactory nozzle clogging resistance and ejection stability and also for imparting satisfactory color density and gloss to prints. To meet these properties, it is required to disperse the pigment in the form of as fine particles as possible in the ink.
Dispersion of a pigment is an operation to mill or crush the pigment, which exists as aggregates, into primary particles or into a form close to primary particles. An ordinary dispersing method of a pigment is to mill or disperse a pigment dispersion mixture, which has been obtained by mixing the pigment and a dispersant in a dispersing medium, by making combined use of a disperser such as a ball mill or sand mill and a dispersing medium. Employed as the dispersing medium are beads made of a material such as glass, iron or ceramic and having a diameter of from several millimeters to 1 mm or so. When the bead diameter is reduced, the number of beads per unit volume increases in geometrical progression, leading to a significant increase in the number of collisions between the pigment and the beads so that the pigment can be dispersed as fine particles.
When the bead diameter becomes excessively small, however, the weight per bead becomes small so that the impact force applied to the pigment upon collision of the bead against pigment becomes far weaker. As a result, a longer time is required for the dispersion of the pigment or the dispersion of the pigment into fine particles cannot be achieved. In addition, it is impossible to separate the beads and the pigment dispersion mixture from each other in the disperser. With the foregoing in view, beads of from 0.2 mm to 1.0 mm in diameter are, therefore, used especially upon dispersing the pigment into fine particles.
A color filter useful in a liquid crystal display or the like is primarily fabricated these days by a process that makes use of a pigment dispersion, specifically by applying a color composition, i.e., a so-called color resist to a substrate by a coating process such as spin coating, an electrode position process or a transfer process to form a color film, exposing the film to light through a photomask, and developing the thus-exposed film to patternize the color film into a color pattern (pixels). In general, a color resist is prepared by dispersing a pigment in an acrylic polymer and then adding a monomer, a photopolymerization initiator and the like into the resultant pigment dispersion (which is also called “base color”) to impart photosensitivity to the base color. Employed as the pigment can generally be a color filter pigment produced for color filters and having primary particle sizes of from 20 to 50 nm. The pigment, an acrylic polymer, a high-molecular dispersant and a solvent are premixed, and by a bead mill containing beads having diameters of from 0.2 mm to 1 mm, the pigment is dispersed into a base color for a color resist [Norihisa NOGUCHI: “Ink Seizo to Seisan Gijutsu (Ink Manufacture and Production Technology)”, Journal of Japan Society of Colour Material, 71(1), 57-67 (1998)].
Keeping step with the recent move toward inkjet printers of higher resolution and higher printing speed, there is an increasing demand for pigment inks having still higher ejection stability, color density and gloss. It has, therefore, become necessary to further reduce the particle size of a pigment dispersed in an ink. Described specifically, a reduction in the particle size of the pigment in the ink leads to a rise in the density of a color to be developed from the ink and also to inhibition of the scattering of light by pigment particles in the resulting ink film so that the thus-formed image can be provided with a higher gloss. With the above-described process that makes use of beads having diameters in the range of from 0.2 mm to 1.0 mm, however, it is difficult to divide a pigment further into particles finer than the current particle size. The division of a pigment into still finer particles is feasible by adopting tougher conditions for the operation of a disperser, for example, by making longer the residence time of a pigment dispersion mixture in a disperser and/or by increasing the circumferential speed of the disperser. These approaches have, however, developed one or more new problems in that the resulting dispersion is provided with considerably-reduced storability or a dispersion is available with lowered productivity.